I mean there's all kinds of reasons why content is cut from the final release. Maybe it wasn't as far along as they would have liked and so didn't release it. Maybe their self-professed "high standard of quality" thought it was too simple and so went with the more emotionally complicated "Avernus or Death route". Maybe they thought the people would like the tragic ending more and were blindsided when everyone hated it so much. Maybe they thought that Dammon just coming in at the end and going "Wait! I figured it out!" would be kinda anticlimactic and so scrapped it. Maybe it's a combination of all of the above (which is my guess).
It's fairly obvious that a path to fix Karlach's engine was at least intended and partially implemented, what with Enriched Infernal Iron (which they still haven't removed despite being completely useless) and the Steel Watchers and the Gondians and Dammon's line about magical coolant and all of the items referencing their cooling powers and etc etc etc. The point is now they know, I mean how could they not, right?
